Output buffers hold data awaiting transmission. The output buffers transmit the data upon receipt of an appropriate signal. The buffers may provide the data to terminals, pads, transmission lines, busses, traces, receiving circuits, etc. (referred to generally hereinafter as “receiving components”).
One critical property of the output buffer is the non-linearity of its output response characteristic. When a linear input (e.g., voltage ramp) is applied to an input an output gain is not constant. The linear input may be provided by a transistor (e.g., pull up transistor). The non linear output gain often has one or more low gain regions separated by a high gain region. Crossovers between the low gain and high gain regions is where the gain is equal to one and is known as Unity Gain (UG). Pre-boosting and post-boosting take advantage of the non linear gain property by rapidly transitioning the input in the low gain region and exerting edge rate control in the high gain territory.
As pre-boosting and post-boosting are dependent on transitions between low and high gain regions a determination of the crossover between these regions is desired (e.g., determination of UG). However, UG may fluctuate, for a number of reasons, and is usually process, voltage and temperature (PVT) dependant. Pre-boosting to the correct UG results in balanced falling and rising transitions. Over pre-boosting speeds up the output transition because the high gain region is encroached before UG, while under pre-boosting slows down the output transition because it takes time for edge rate control to drive past the UG.
One method for pre-boosting includes use a self-timed circuit. When the input transitions, the pullup leg of the transistor is enabled for a time delay provided by an inverter chain. The preboosted level depends on the node capacitance C at the output, the strength of the pullup/pulldown legs and the time delay. None of these parameters are PVT compensated. As a result, under or over pre-boosting may occur as process, voltage and temperature vary. The under or over pre-boosting causes unbalanced falling and rising transition at the output.